Move TaskContext into TCB instead of kstack.

os/src/task/task.rs

@@ -1,13 +1,9 @@
+use super::TaskContext;
+
 #[derive(Copy, Clone)]
 pub struct TaskControlBlock {
-    pub task_cx_ptr: usize,
     pub task_status: TaskStatus,
-}
-
-impl TaskControlBlock {
-    pub fn get_task_cx_ptr2(&self) -> *const usize {
-        &self.task_cx_ptr as *const usize
-    }
+    pub task_cx: TaskContext,
 }
 
 #[derive(Copy, Clone, PartialEq)]